(1) Field of the Invention
The present invention relates to a solid-state image sensor, and more specifically to a structure of a pixel.
(2) Description of the Related Art
In recent years, MOS image sensors have been gaining more attention in the field of solid-state image sensors, because the MOS image sensors are more advantageous in downsizing and reducing power consumption than charge-coupled device (CCD) image sensors.
The MOS image sensors have a structure in which a plurality of pixels are arranged in matrix on a p-type semiconductor substrate (or a p-type well). Signal charges in the pixels, each generated according to an amount of light received in each pixel, are scanned by low and column and read out sequentially, and then image data for one frame is outputted. Each pixel includes a photoelectric converting layer, a gate electrode, and a read out region.
The photoelectric converting layer is an n-type region formed in the p-type semiconductor substrate. A depletion region, where carriers of the both regions disappear after getting combined to each other, is formed in vicinity of a border between the p-type region and the n-type region. In other words, in the depletion region, there is no electron in the n-type region and no hole in the p-type region. Thus, a potential at the n-type region is relatively higher than that at the p-type region in the depletion region, which indicates that an internal electric field is generated in the depletion region.
With a sufficient amount of incident light, electron-hole pairs are created in the semiconductor substrate. When the electron-hole pairs are created in the depletion region, electrons and holes are drifted in opposite directions due to the internal electric field. The electrons are collected in the n-type region, and then become a signal charge.
The gate electrode controls transferring the signal charge from the photoelectric converting layer to the read out region. When a positive gate voltage is applied to the gate electrode, a channel is formed at a region of the substrate directly below the gate electrode. The signal charge is transferred through this channel from the photoelectric converting layer to the read out region.
In general, many crystal defects can be observed in vicinity of a surface of the substrate due to an impurity implantation. In such a part in the substrate, the electron-hole pairs are created by thermal excitation. Because the excitation of these electrons is not based on the amount of received light, these electrons become a noise component when collected to form the signal charge, and result in defects known as white spot defects.
As one solution to eliminate such white spot defects, Japanese Laid-Open Patent Application No. 2001-345437 discloses a solid-state image sensor having an impurity layer positioned at a surface of a substrate and a photoelectric converting layer positioned directly beneath the impurity layer. The impurity layer is formed by implanting a p-type impurity at a high density. Increasing the impurity density in the impurity layer keeps the depletion region in the photoelectric converting layer from spreading to the surface of the substrate. Therefore, even if the electrons are thermally excited in vicinity of the surface of the substrate, the electrons are not collected to the depletion region. As a result, it is possible to eliminate the white spot defects.
The above conventional technique, however, also presents a new problem as described below, even though the problem caused by the white spot defects can be solved by the conventional technique.
When the density of the p-type impurity is high, it becomes hard to form the channel sufficiently at the region of the substrate directly below the gate electrode, and not all of the signal charge can be transferred. As a result, an amount of the transferred signal charge decreases, and a signal-noise ratio deteriorates. This hinders production of the image sensors with high-sensitivity.
Increasing the gate voltage makes it possible to form the channel sufficient to transfer all of the signal charge. However, this increases an amount of power consumption as well, which hinders production of the low-power image sensors.